Hi, the Dawn detergent should be mixed with water, look into old posts for the "magical" formula. When bathing, start with the soap at the neck, this sort of creates the barrier so that the fleas don't hide out in the head.
I have started using a shampoo called Mink Sheen,
More Product Descriptions it smells good, it protects for days, it kills fleas on them. I also read a thread that said you can put 20 drops of lavendar, rosemary into a 16 oz dog shampoo bottle and you get great protection. Good luck.